For producing a woven cloth, the warp threads of the warp must be divided into an upper shed and a lower shed starting from the so-called binding point to the weaving reed, in order to carry out a weft insertion. This is achieved by heald shafts in heald shaft looms and by harness cords in jacquard looms.
The resulting angles between the warp threads bounding the loom shed in this context are directly dependent on the position of the backrest and the stroke of the heald shafts (harnesses). These angle relationships of the loom shed are important for the weaving operation and are different for different woven cloths. That generally means that the position of the backrest must be changed and newly adjusted in connection with a change of the woven cloth to be produced (product article change). A similar consideration often pertains also to the so-called warp stop motion, of which it is the object to monitor the warp for warp a thread breaks and to stop the loom upon the occurrence of a warp thread break.
An uncontrolled tensioning unit for the warp of a weaving loom is known from German Patent Publication DE 195 38 121 C1 and corresponding U.S. Pat. No. 5,755,268 (Arndt et al.), which basically shows how the tensioning unit, which includes a backrest roller (backrest beam), is supported on the loom frame in a manner so as to be adjustable in a horizontal and vertical direction. The tensioning unit is supported on both sides on two carriage-like components, which are longitudinally slidably guided on two guide parts, which in turn are supported on the side cheeks of the loom frame so as to be tiltable about a common horizontal axis. Clamping screws cooperating with corresponding elongated or slotted holes make it possible to position the tensioning unit and therewith the backrest roller into the optimum desired position for the respective woven cloth to be produced. In this context, the adjustment of the tensioning unit is carried out by hand.
Because this adjustment of the backrest beam requires a certain degree of care, a loom known from U.S. Pat. No. 5,261,463 (Sato) has already been equipped with an arrangement for monitoring or checking the correct positioning of the tensioning unit for the warp. The backrest beam of the loom is supported on the machine frame on each side of the machine by means of two carriage-like bearing parts that are movable at right angles to each other. Clamping screws reaching through elongated or slotted holes make it possible to move the backrest beam, which is adjustable in the horizontal direction and in the vertical direction in this manner, into its respective desired position, and then to fix it in this position. The operation of the clamping screws and the adjusting of the backrest beam from the actual position into the desired position are carried out by hand. Non-Contacting or contactless optical sensors are allocated to the carriage-like bearing parts on each side of the machine, wherein these sensors determine the horizontal and vertical position of the backrest beam and provide corresponding data into a control unit, where these data are stored and then caused to be displayed on a display screen. An operator can thereby follow the adjustment process on the display screen, whereby the attainment of a prescribed desired position is indicated by light signals. Nonetheless, the adjustment of the backrest beam comprising two rollers remains time consuming, because on each side of the machine a plurality of clamping screws must be loosened by hand and then must again be retightened by hand after the similarly manually conducted process of moving the backrest beam into the respective desired position. As a side comment, a similar consideration pertains also to the warp stop motion, in case such an arrangement is provided.